This invention relates to a system for controlling a speed of an induction motor comprising a speed sensor with both a voltage applied across the induction motor and a frequency thereof, and more particularly to the limitation of a torque or an output from an induction motor controlled by such a speed control system.
Conventional speed control systems of the type referred to have comprised: an error amplifier for producing an error corresponding to the difference between the actual speed of the induction motor and the command speed therefor; a slip frequency limit generator for generating a slip frequency limit in response to the actual speed of the induction motor, and a limiter for limiting the error of the actual motor speed with respect to the slip frequency limit. Then, the adder adds an output from the limiter to the actual motor speed value and applies the resulting sum to the inverter for use as a command frequency. The inverter supplies a predetermined voltage at the command frequency to the induction motor to control the latter.
On the other hand, induction motors have the constant torque range forming the lower speed portion of the variable speed range and the constant output range forming the higher speed portion thereof. In conventional speed control systems such as described above, the induction motor has been, in many cases, controlled so that a voltage applied thereacross is approximately proportional to a frequency thereof in the constant torque range and that the square of the voltage is approximately proportional to the frequency in the constant output range. With the induction motor controlled as described above, the relationship between the rotational speed thereof and the slip frequency limit has been actually complicated and also it has been fairly troublesome to establish that relationship.
If the rotational motor speed and slip frequency limit do no fulfill their established relationship, then a current through the inverter has exceeded its permissible maximum magnitude resulting in the possibility of causing a fault thereon. In order to avoid the occurrence of the fault, the current limiting function might be imparted to the inverter. This has resulted in the disadvantage that, with the current limitation effected prior to the frequency limitation, the induction motor reduces in torque or output as compared with only the frequency limitation being effected even though the frequency limitation would have been effected after the current limitation. This is because the current limitation decreases the rotational speed of the induction motor.
Accordingly, it is an object of the present invention to provide a new and improved speed control system for an induction motor free from the disadvantages of the prior art practice as described above by limiting a slip frequency of an induction motor in the current limiting mode of operation.